In recent years, printing employing a CTP system has been conducted in printing industries, accompanied with the digitization of printing data. A printing plate material for CTP, which is inexpensive, can be easily handled, and has a printing ability comparable with that of a PS plate, is required.
A versatile processless printing plate has been sought, which has a direct imaging (DI) property not requiring any development employing a specific developer, can be applied to a printing press with a direct imaging (DI) function, and can be handled in the same manner as in PS plates.
A thermal processless printing plate material is imagewise exposed employing an infrared laser with an emission wavelength of from near-infrared to infrared regions to form an image. The thermal processless printing plate material employing this method is divided into two types; an ablation type printing plate material and an on-press development type printing plate material with a heat melting image formation layer.
Examples of the ablation type printing plate material include those disclosed in for example, Japanese Patent O.P.I. Publication Nos. 8-507727, 6-186750, 6-199064, 7-314934, 10-58636 and 10-244773.
These references disclose a printing plate material comprising a support, and provided thereon, a hydrophilic layer and a lipophilic layer, either of which is an outermost layer. When a printing plate material is imagewise exposed in which the hydrophilic layer is an outermost layer, the hydrophilic layer is removed by ablation to reveal the lipophilic layer, whereby an image is formed. This printing plate material has problem that the exposure device used is contaminated by the ablated matter, and a special suction device is required for removing the scattered material. Therefore, this printing plate material is low in versatility to the exposure device.
A printing plate material has been developed which is capable of forming an image without ablation, and does not require development treatment employing a special developer or wiping-off treatment. There is, for example, a printing plate material for CTP as disclosed in Japanese Publication Nos. 2938397 and 2938397, which comprises an image formation layer containing thermoplastic particles and a water-soluble binder and which is capable of be developed with a dampening solution or printing ink on a printing press (on-press development).
Generally, a light-to-heat conversion material (generally colored) needs to be incorporated in the image formation layer, and is transferred to a printing ink or a dampening solution during printing, which causes problem of contaminating a printing press during on-press-development.
As a method for preventing the contamination of a printing press which is caused during on-press development, a method has been proposed which employs, as a hydrophilic support, a support in which a hydrophilic layer containing a light-to-heat conversion material is provided on a substrate. Such a hydrophilic layer makes it possible to eliminate the light-to-heat conversion material from the image formation layer. Further, this method can achieve high sensitization without contaminating a printing press, since it can increase the amount of the light-to-heat conversion material in the hydrophilic layer or incorporate a slight amount of the light-to-heat conversion material in the image formation layer.
In order to improve function of the hydrophilic layer of the printing plate material for CTP, for example, a printing property and an image retention property, there is proposed a hydrophilic layer (see, for example, Japanese Patent O.P.I. Publication No. 2000-225780) containing porous inorganic fillers with a particle size not more than 1.0 μm as well as the light-to-heat conversion material, or a hydrophilic layer (see, for example, Japanese Patent O.P.I. Publication No. 2002-370465) containing plural irregularity structure-forming inorganic fillers and an inorganic binder with high porosity as well as the light-to-heat conversion material.
A processless printing plate material for CTP is required which provides higher sensitivity and better printing performance.
However, even a printing plate material comprising the hydrophilic layer described above has been difficult to improve printing properties including sensitivity and printing durability.
In a processless printing plate material for CTP capable of being developed on a press, a layer with scratches cannot be developed, which may cause stains on the background of prints. A method solving this problem is also required.